A fluorescent lamp fixture requires a ballast for providing the starting and operating current to one or more fluorescent bulbs. These ballasts utilize a transformer, having a magnetic core and coil assembly including a pair of coils. The magnetic core is usually laminated, i.e. made up of a number of thin ferromagnetic plates insulated from one another. The coils are commonly referred to as a primary and a secondary winding, with each winding formed by winding a thin copper wire on a spool or bobbin made from plastic or some other insulating material. The coils are disposed on a central leg of the magnetic core, with each bobbin having a hollow center for fitting onto the central core leg. Two outside core legs are added to surround the windings to maximize flux concentration. During transformation, energy is transferred from the primary winding to the secondary winding by electromagnetic induction.
Ballast transformers may include one or more ferromagnetic shunts positioned between the primary and secondary windings. Such shunts increase the leakage reactance of the transformer by providing a flux leakage path between the primary and secondary windings. This flux leakage path is controlled by the air gap between the shunt and the core legs. The shunt therefore diverts a portion of the magnetic flux generated by the primary winding to prevent coupling with the flux generated by the secondary winding. Shunted transformers also limit the short circuit current to a greater degree than those transformers that do not include such shunts, with the current reduction varying with the spacing (air gap) between the shunt and the adjacent core.
Generally, a U-shaped shunt, composed of a plurality of planar ferromagnetic laminations, is inserted between the windings, with the upright legs of the shunt wrapped with tape to provide a snug fit between the center and end core legs. The shunt laminations may be held together by the tape, or may be bound with adhesives or fasteners. The thickness of the tape determines the gap between the shunt and the core, with compression of the tape maintaining the proper gap over the life of the ballast.
One problem with the current method for including shunts in transformers is that with time, the tape may deteriorate or cold flow, allowing the shunt to shift position. Not only does this alter the short circuit current, but in typical ballast transformers used for electrical lighting, the shunt may vibrate, causing a low level hum which is considered a nuisance.